topojson-server | Convert GeoJSON to TopoJSON | Map library

I've got the lovely topojson library working for me in a Rails app locally:

https://github.com/topojson/topojson

and it's allowing us to show some nice geoJSON in a google map like so:

I show it with the map.js file exposed, since that seems to be where the trouble is when we deploy to heroku. On heroku the map.js file gives an error on chrome:

maps-....js:27 Uncaught SyntaxError: Unexpected token export

and a similar error (although on a different line) on firefox:

SyntaxError: export declarations may only appear at top level of a module maps-...js:1

I've been in touch with heroku support who suggested locking down our npm version, as it seems like the minified js file is ending up with slightly different content locally than on heroku. I note that I've done everything I can to replicate production mode locally, clobbering and recompiling the static assets etc. and running like so: